In order to strengthen holes in metal aerospace structures such as wing skins, spars and ribs, the holes are cold worked. The cold working or expansion of a hole in a metal component creates a compressive stress zone around the hole that causes a slight increase in hardness and improves fatigue resistance of the material. Conventional cold working techniques include manual procedures and automated processes. Automated methods have utilized stationary cold working mandrels and movable workpieces. However, such methods have several disadvantages, including the requirement of moving relatively large workpieces in relation to stationary mandrels.
An approach to address these problems would be to provide a movable mandrel and a stationary workpiece. However, the use of a movable mandrel raises several issues such as misalignment between the mandrel and the workpiece. For curved workpieces, such as aluminum wing skins, exact alignment of the mandrel with the axis or vector of the hole is difficult. Also, the mandrel head must be flush with the surface of the workpiece, otherwise the mandrel may be pulled toward the workpiece and broken during the cold working operation.
The present invention has been developed in view of the foregoing and to address other deficiencies of the prior art.